1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus imaging an inspection target by using an ultrasonic wave.
2. Description of the Related Art
For example, an ultrasonic imaging apparatus for medical use and so on generates an image by using pencil beams (ultrasonic beams with a narrow angle of beam spread) generated by two-dimensional or three-dimensional scanning of a microscopic piezoelectric element. In order to generate a three-dimensional image here, the scanning has to be repeated in a three-dimensional space an enormously large number of times such as several hundred times to several thousand times, for instance.
Meanwhile, there has been developed a method for generating an image by transmitting ultrasonic waves with a wide angle of beam spread from piezoelectric elements arranged in matrix and applying aperture synthesis to received echo signals. An ultrasonic wave is transmitted from one of the piezoelectric elements as a transmission point and is received by the other piezoelectric elements. The transmission and reception are repeated approximately several ten times while the transmission point is changed, so that a three-dimensional image can be generated.
Here, when ultrasonic waves with a wide angle of beam spread are transmitted from the piezoelectric elements, there is a limit to improvement in resolution of an ultrasonic wave image. In the aperture synthesis, resolution is higher as the transmission/reception angle of beam spread of the ultrasonic waves is wider. To widen the angle of beam spread, the piezoelectric elements need to be made smaller. However, size reduction of the piezoelectric elements results in lower sensitivity, which may lower the resolution of an ultrasonic image all the more.
As a solution, there has been proposed an ultrasonic imaging apparatus that causes the transmission of an ultrasonic wave from one virtual point sound source by controlling the transmission timings of ultrasonic waves from a plurality of piezoelectric elements (see JP-A 2000-28589 (KOKAI)). Because the sensitivity does not lower even with the size reduction of the piezoelectric elements, resolution can be improved.
However, it has been difficult to generate an ultrasonic beam with a narrow angle of beam spread by using this ultrasonic imaging apparatus.